Enteric fever caused by Salmonella enterica serovar Typhi (S. Typhi) poses a significant threat to public health, and is responsible for an estimated 21 million illnesses and 200,000 deaths annually. Increasing antibiotic resistance among S. Typhi isolates has further exacerbated the threat from this infectious agent. To develop improved broad spectrum vaccines against S. Typhi, a better understanding of host immune responses that are responsible for protection is necessary. The role ofthe resident gut microbiota in this process is only now just beginning to be explored, even though it has been established that certain members ofthe gut microbiota are intimately linked to mucosal immunity. Previous studies from Drs. Fraser and Sztein carried out under the current CCHI funding have shown that multi-phasic CMI responses following immunization with oral, live-attenuated S. Typhi correlate with the presence of a high diversity gut microbiota, and that over two hundred members within the order Clostridiales differentiate multi-phasic and late responders. A human challenge model of wild-type (wt) S. Typhi infection has recently been established by the Oxford Vaccine Group, and a study has been performed to determine the protective efficacy of the licensed oral, live-attenuated vaccineTy21a and the novel ZH9 vaccine candidate using this model. We propose an innovative and integrative study that will leverage the availability of these samples to characterize the structure and function of the gut microbiota following exposure to S. Typhi and will allow us to further investigate the central hypothesis that mucosal immune responses (to be determined in Research Project 1) are intimately linked to the gut microbiota and, that together, these might determine susceptibility to or protection from typhoid disease. The results from this project will be validated using in vitro systems in Research Project 3. In addition, we will also test the hypothesis that robust immune responses in children immunized with Ty21a correlate with the presence of a high diversity gut microbiota. Immune data and microbiome data will be integrated in collaboration with the Bioinformatics and Biostatics Core to provide a holistic view of the interplay between the mucosal immune system and the gut microbiota.